1. Field of the Invention
The present invention relates to Magnetic Resonance Imaging (MRI) devices, and more particularly, to MRI devices designed to image a particular body part or region. The present invention utilizes fixed local coils for receiving electromagnetic signals from resonating nuclei produced by a whole body MRI scanner system or the like to produce high quality images.
2. Discussion of the Prior Art
MRI is a well known technique wherein an object, animate or inanimate, which is placed in a spatially varying magnetic field is subjected to a pulse of Radio Frequency (RF) radiation, and the resulting nuclear magnetic resonance spectra are combined to give cross-sectional images of the object. The MRI technique is possible because the human body contains an abundance of hydrogen atoms, whose nuclei are protons, in its tissues, and these protons respond to electromagnetic manipulation, which is obviously essential in MRI. Generally, an MRI apparatus operates by the application of an RF excitation field in the presence of other magnetic fields and the subsequent sensing and analysis of the resulting nuclear magnetic resonance produced in the body.
Any nucleus which possesses a magnetic moment tends to align itself with the direction of the magnetic field in which it is located. Accordingly, when a substance such as human tissue is subjected to a static magnetic field, the individual magnetic moments of the protons in the tissue attempt to align with this polarizing magnetic field. However, the protons precess around the direction of the field at a characteristic angular frequency, known as the Larmor frequency, which is dependent on the strength of the magnetic field and the properties of the specific nuclear species. Once in the polarizing magnetic field, the alignment of the protons exist in one of two possible energy states which describe the spin angular momentum of the protons. Classically, the protons precess, that is, each proton's axis of rotation generally describes a cone and tends to turn at an angle relative to the direction of the applied polarizing magnetic field. The protons precess in a random order in terms of the phase of rotation. A net macroscopic magnetic moment is produced in the direction of the polarizing field, but the randomly orientated magnetic components in the perpendicular or transverse plane to the polarizing magnetic field cancel one another. If however, the substance or tissue is subjected to an RF radiation pulse which is in the plane transverse to the polarizing magnetic field and which is at or near the Larmor frequency, the net aligned moment may be rotated or tipped into the transverse plane to produce a net transverse magnetic moment which is rotating or spinning in the transverse plane at or near the Larmor frequency. Essentially, the pulse of RF radiation is utilized to achieve resonance and produce a phase coherence such that the precessing protons are no longer random in phase, but rather at a single phase orientation. The degree to which the net magnetic moment is tipped, and hence the magnitude of the net transverse magnetic moment depends primarily on the duration of time and the magnitude of the applied RF radiation signal.
The practical value of the above described phenomenon resides in the signal which is emitted by the protons when the RF radiation pulse is terminated. Basically, a measurement is performed on the resonance signal emitted as feedback by the protons during the period when their magnetic moments tend to re-align themselves with the polarizing magnetic field. The measured signal is then processed in order to extract therefrom cross-sectional images of the tissues or organs under examination. Essentially, as the protons are precessing and travelling back towards alignment within the polarizing magnetic field, they are "cutting" the plane of receiving antenna which is part of the MRI device; accordingly, a current is induced in the receiving antenna as explained by Faraday's Law. From this induced current or Electro-Motive Force (EMF) signal, a map of the proton density of the tissue being imaged and its relaxation times, which is the time necessary for the protons to realign themselves with the polarizing magnetic field is generated. This feedback signal is processed and is ultimately transformed into a series of images of the tissue.
Various types of receiving antennas or coils have been designed for MRI applications. The most commonly utilized antenna is the standard sized whole body coil which is dimensioned to be disposed around the entire body of the patient to be imaged. Due to the standard sizing, a significant void or empty region is defined between the coil and the portion of the patient to be imaged. As the imaged portion of the patient becomes a smaller fraction of the coil volume, the signal-to-noise ratio decreases, thereby degrading the image quality. In addition, the coil receives resonance signals from over a significantly larger area than the region of interest. This sensitivity to extraneous information degrades the spatial resolution and increases aliasing in the two and three dimensional Fourier Transform methodology utilized in processing the resonance signals.
The typical form of the MRI device employing the above described whole body coil is a tubular member or tunnel into which the individual is placed. This type of arrangement, when used for applying an RF field, does not result in a high quality homogeneous field, nor is it efficient in the generation of the field. In addition, many individuals do not feel comfortable when placed in a tube, especially, those individuals who exhibit claustrophobic tendencies.